Multichannel carrier communication system



l1, 1945- P. K. CHATTERJ'EA Erm. 2,390,641 MULTICHANNEL CARRIER COMMUNICATION SYS'TEM Filed Feb. 1s, 194s s sheets-sheet 1 Dec. 1l, 1945- P. K. CHATTERJEA rm. 2,390,641

MUIZJIICHAMIIIEL` CARRIER COMMUNICATION SYSTEM Filed Feb. 1.6,4 1943 3 Sheets-Sheet 2 SUB 57A 7'/0N Mo. 2

De 11, 1945- P. K. cHATTERJl-:A TAL 2,390,541

MULTICHANNEL CARRIER COMMUNICATION SYSTEM 3 Sheets-Sheet 3 Filed Feb. 16, 1943 asp.

PUL .SE

Inventor /fklnev l 07'/ f Atlor y V l for the provision of Patented Dec. 11, 1945 MULTICHANNEL CARRIER COMMUNI- CATION SYSTEM Prafulla Kumar Houghton,

Standard Telephones Application February 16, 1943, Serial No.

Chatteriea and Leslie Wilfred London,

England, asslgnors to and Cables Limited, London, England, a British company In Great Britain March 13, 1942 (Cl. Z50-9) 8 Claims.

The present invention relates to radio communication systems and it has for its object to provide a radio communication system for communication between a central station and a plurality of stationary or mobile substations.

All the requirements of a flexible intercommunication system connecting a number of flxed positions are fully met by a modern telephone system employing cables. Where, however, all the points to be interconnected are mobile, the employment of radio as the inter-connecting link becomes essential. Where the requirements are a number of separate channels of communication between two main centres, the substitution of radio transmission for cables presents few problems as the modulation frequency spectrum may readily be subdivided as in the case of carrier telephony and only two carrier frequencies may be required. This method becomes impracticable, however, in the case where the termination of each channel is at a different location as, although the transmission from the main or central station remains the same, the return channel from each receiving location or substationwould have to be made on separate carrier frequencies with corresponding receiving equipment at the central station. Such a system is therefore only slightly less complicated than the employment of self-contained radio links for each location or substation.

In the systern'according to this invention about to be described radio carrier transmission is sub-` stituted for cables whilst retaining all the principal advantages of the latter systems and in addition the whole system may be made more flexible if desired.

According to one aspect of the present invention in a radio communication system for communication between a central station and a-plurality of stationary or mobile substations, at the central station each of a plurality of signal channels is arranged to modulate its respective subcarrier frequency, and all the sub-carrier irequencies are combined together to modulate a principal carrier frequency, one or both of the side-bands of the modulated principal carrier frequency being radiated with or without the principal carrier frequency and at each substation the said side-band or side-bands with or without the principal carrier frequency are received and one or more of the modulated subcarrier frequencies is/are obtained therefrom.

According to another aspect of the invention, in a radio-communication system for communication between a. central station and a plurality of mobile substations, at the central station, a plurality of signal channels each modulates its respective sub-carrier frequency and all the subcarrier frequencies are combined together to modulate a principal carrier frequency, one or both the side-bands of the modulated principal.

carrier frequency with or without the principal carrier frequency being radiated and at each substation the said side-band or side-bands with or without the principal carrier frequency is/are received and one or more of said sub-frequencies obtained .therefrom and also at a substation, the said principal carrier frequency is generated or derived and modulated by a sub-carrier frequency modulated by a signal wave for communication to the central station, the modulated principal carrier frequency being radiated from said substation, and received at the central station and the modulated sub-carrier from the said substation obtained from said modulated principal carrier frequency received at said central station. i

In carrying the invention into practice a wave rich in harmonics is used to modulate an oscillator, and the side band frequencies so produced are then separated so as to provide individual sub-carriers, one for each required channel. Means are provided for modulating each subcarrier and the modulated sub-carriers are then combined and, as a whole, heterodyned up to the required transmission frequencies by a local oscillator of the principal carrier frequency, say fc or fe may be modulated and the requisite sidebands selected. The output is therefore a number of modulated signals, rigidly related to each other which may all be transmitted by the same transmitter and aerial array.

The ,receiving substations for the channels consist of similar equipments comprising an amplifler and a local oscillator of, or means for deriving frequency fc, which is caused to heterodyne the received signals so producing the initial series of modulated sub-carriers generated at the transmitter and from these sub-carriers the respective one is selected "and, demodulated in known manner.

For transmitting a signal in the reverse direction, namely from a substation to the central station thereby providing a two-way communication link, an oscillator of the same sub-carrier frequency as that providing the particular received channel in question is incorporated in the receiver, and then by suitable switching arrangements this frequency is modulated by th'e signal and heterodyned by or caused to modulate osciltion. By suitable switching arrangements the same selection circuitsare employed to select the appropriate signal which is then demcdulated. llt is therefore apparent that each channel can be used independently, the selection equipment at the central station being arranged and operated similarly to a normal telephone exchange panel, the operator of which has no adjustments to make in connection with the transmission.

A radio system according to the invention is particularly applicable where the area of operation is limited, so that comparatively high frequency radio transmission can be employed, with low power radiation, as the individual receivers are then correspondingly simpler due to the possible dual use of high frequency sections. All the filtering of channel sub-carrier frequencies is at radio frequency level with corresponding saving in size and weight of equipment as compared with low frequency filtering.

In the event of an emergency such as might occur should the central station be put outof action, means may readily be provided for permitting intercommunication directly between individual sub-stations.

The invention will be made clearer in the following description of some embodiments of the invention illustrated in the accompanying drawings and given by way of example.

In the drawings:

Figure 1 shows diagrammatically the circuit arrangements of the central station and two substations;

Figure 2 is an explanatory diagram showing a channel frequency distribution in the frequency spectrum; p

Figure 3 is an explanatory diagram;

Figure 4 is an explanatory diagram showing the channel frequency distribution in the frequency spectrum of a second embodiment of the invention;

Figure 5 shows diagrammatically the circuit arrangements of a central station of a second embodiment of the invention;

Figure 6 shows diagrammatically'the circuit arrangements of a substation of the second embodiment;

, Figure '1 shows diagrammatically'a modification of the arrangements shown in Figure 6;

Figure 8 is an explanatory diagram andV Figure 9 shows diagrammatically a further modification of the arrangements shown in Fig- `ure 6.

Referring now to Figure 1 there is shown the circuit arrangements for the case in-which only quency could be derived. y Only two substations are shown, but it will be understood that more than two substations may be employed.

It has sofar been implied for the sake of simpliclty, that the principal carrier frequency employed at each substation, is the same as that used at the central station. A further modification of .'this which may be incorporated-in the case shown in Figure 1 and which does not alter the circuit arrangements involved but only the frequency, consists in `using a, different frequency for each substation principal carrier frequency and then using a corresponding sub-carrier frequency so as to transmit a signal corresponding to the central station alternative side band frequency to that transmitted by the central station and received by the substation. This has the advantage that the selectivity of the high frequency stages at thev central station has only to be sufficient to prevent unwanted side-band break through which would show itself as interchannel interference, the possible low frequency beat occurring betweenI received and local principal vcarrier frequency being completely eliminated.

The required selectivity of the central station high frequency circuits will depend on the arrangement of aerials, and at the substations on the possibility of a signal from another channel appearing as an image signal, but this may be completely avoided if too large a number of channels are not employed.

Referring to Figure 1. the central station consists ofv generator I of pulses which are rich in harmonics of the pulse repetition frequency, and are used for modulating an oscillator 2 from the output of which by means of selective devices are produced the sub-carrier frequencies. For transmission from the central station these frequencies are selected by lter amplifiers 3, 31 and modulated at 5 and 51 by, for example, a signal from a microphone 20, 201 and then A'fed to a modulator unit 6 to which is also fed the output consisting of the principal carrierv frequency fc from a high frequency oscillator 8. The required sideband components are then selected in any known manner and amplified by l before being fed to aerial TA1 for radiation.

It will be seen from the diagram that for this operation all switches operating to one channel i. e. II, I3, I5, I'I or I2, I4, I8 and I8 are in position T and for reception ofa signal they will be changed to position R when the operation would be as follows? Incoming signals, which are arranged to have frequencies corresponding to images or opposite sidebands to those transmitted relative to the frequency of oscillator 8, are received at aerial RAID and amplified at l0, then heterodyned at 9 by oscillator 8 so reproducing sub-carriers which may then be selected by filter amplifiers 3, 31 after which they are demodulated at 4 and 41 and fed to, for example, headphones I9, |91 after being amplified by 5, 51, which are now used as low frequency amplifiers.

The operations of all substation units are identical and differ only in the actual frequencies employed. So referring to substation No. 1 the signals radiated by the central station are received by aerial RA2I and amplified by amplifier 2| after which they are heterodyned by oscillator 23 at 22 so producing asubcarrier frequency corresponding to the required channel which is selected by amplifier filter 24, demodulated by 25 and fed to headphones 30 via for which operation switches 3i, 32, 33 and 34 are switched to position R.

low frequency amplifier 2Bv For transmission from substation No. lback to the ccntraiststion these switches are. moved to position T and a signal obtained for example from microphone 2l is after amplification by ampliiler 2l caused to modulate an oscillator 21, the frequency of which is such that when heterodyned by or caused to modulate the output of oscillator 23 at 22 produces a side-band frequency equal to that required to be received by the central station for that particular channel allotted to substation No. l.. This is then selected and amplined by amplifier beforetransmission by aerial TAIR.

In -actual practice appropriate switching arrangements may be included in amplifier 2| for performing the function of 28;

If oscillator 22 is made to generate the same frequency as the central station principal carrier frequency then the oscillator 27 will be the same S'ffrequency as the corresponding central station sub-carrier -and as the frequency at which 24 is selective so that, if desired, the output of oscillator 21 may be amplified by 24 prior to modulation by output of 29 and 2B.

A schematic representation or the frequencies involved is shown in Fig. 2 with reference to sections of Fig. l.

In Figure l2, fp represents the pulse repetition frequency of the pulse generator I (Figure 1) and 2f, 3f etc. are the harmonic frequencies of fp: These pulses are applied to modulate oscillator 2 (Figure l) and produce the sub-carrier frequencies f., fn, fir, fis, f2s etc. which are modulated by respective signals to produce the modulated sidebands si, s2. These modulated subcarriers si, s2

' then modulate the principal carrier frequency fc and each produces two side-bands of modulation products fri-si and fc-sl fs+s2 and fc-s2. Only one each of these pairs of side-bands of each signal, for example, fc4-sl and fc4-32. is transmitted from the central station, and are received at the substations No. 1 and No. 2 as indicated in lFigure 2. One of the side-bands .fe-l-sl is then selected at substation No'. 1 and fc4-s2 is selected at substation No. 2. For transmission of signals from the 'substations to the central station. the "image" side-bands of (js-I-si) and (fc4-s2) or other frequencies below fc are employed. In order to-utillse the same apparatus for transmitting and receiving, the image frequencies are preferably employed. For example, substation No. 1 receives and selects (fc4-si) and transmits (fc-si) whilst substation No. 2 receives and selects (fri-s2) and transmits (fc-s2). Then assuming that the local oscillator 23 at substation No. i has a frequency ,fm the intermediate frequency on reception will be ,fe-i-si-fm and the sub-carrier frequency at substation No. 1 will be fm-fe-i--sL At substation No. 2, if the local oscillator 23 has a frequency fm, the intermediate frequency on reception will be le-i-s2-fm. In order to keep the possibility of a second channel interference at a minimum, it is advisable to arrange that the signal transmitted back to so producing a series of frequencies as its sidebands separated by 15 kcJ/s. two of which, i. e. the 445 and 480 kcJs. components were selected as sub-carriers and after modulation caused to the central station from any substation does not coincide with the image frequency of any other substation while receiving, for which, in the arrangement shown it is necessary to arrange that frm- (fc-sn) does not become equal to (fc4-sul) ftn' I where n and nl represents any channel.

The actual frequencies employed would be dictated by design and requirements, but in an actual experimental rig up, the pulse repetition frequency was 15 kc./s. so giving a channel separation of this amount of 15 kc./s. 'I'hese pulses were then used to modulate a 445 kc./s. oscillator modulate a 29.555 mc./s. oscillator so producing a side-band of 30.0 and 30.015 mc./s. for transmission. At a substation receiver built foi a subcarrier frequency of 472 kc./s. the local oscillator had to be `30.472=29.528 mc./s. The transmission back from the substation to the central station had to be 29.555-.445=29.l10 mc./s. so that the local sub-carrier oscillator 2l .at the substation was 29.52829.1l0 mc./s.=4l8 kc./s.

In the arrangement now to be described and shown in Figures 5 and 6 the sub-carriers modulate a high frequency principal carrier frequency oscillator as before. but both side-bands are transmitted together with the principal carrier frequency. At each substation this modulated principal carrier frequency when received is rectified so as to give the whole series of sub-carriers, the required one of which is then selected. At each substation means are provided for filtering out the principal carrier frequency for use as the frequency to be modulated by a locally produced sub-carrier for communication back to the central station where it is received and mixes with the modulated principal carrier being transmitted in the aerial circuit. preventing carrier frequency both reception and transmisrectly varying the voltage existing in the main,

an operation which would be equivalent to introducing additional side-band frequency camponents. If the modulation frequency or frequencies of each modulating unit are different then there will be no mutual interference. and any particular modulating frequency may be obtained at any position on the ring main by connecting thereto an appropriate demodulator and selecting the required frequency from its output. Such an arrangement might be illustrated as shown in Figure 3 where 4I is an oscillator, 4 2 and d3 medir--` lating units and 4B a demodulation unit.

Similar conditions may also be accomplished by substituting electromagnetic radiation :for the conductors forming the ring main.

The radiation to be modulated may be regarded as the principal carrier frequency fc and is generated at the central station where, at the same time it may be also modulated by a. number of individual groups of frequencies. The process whereby additional modulation may this carrier is accomplished at a substation by receiving the signal transmitted by the" central station, and by means of a. filter separating the carrier frequency le or generating a similar frequency and then modulating it in the usual way by a sub-carrier before re-transmitting it. This new modulation will therefore cause additional frequency components to appear in the general radiation. and the carrier component fc will'be an addition to the existing fc their relative phases determining whether the resultant amplitude is greater or less than one component of fc; however. this can be arranged to have little effect on be added to the remainder of the system. Demodulation of fo at the central station may be accomplished by normal receiver technique, and an individual subcarrier modulation may then be further selected as required.

Each sub-carrier corresponds to a separate Y communication channel, these sub-carriers being generated, (at the central station) preferably by shown diagrammatically in Fig. 4 where fo represents the fundamental frequency ofthe initial waveform rich in harmonics together with its harmonics 210, 3f@ etc. which are employed at the central station for generating the required number of sub-carriers, one for each communication channel. By modulating a frequency fs, sidebands are generated on either side of fs as shown and the requisite number are then separated by filters to form the individual sub-carrier f1, f2, fa

etc. which may then be individually modulated as desired. This group of frequencies is then caused to modulate the principal carrier frequency fc which forms the signal radiated from the main transmitter. Where a message is being received at the main station on any particular channel, the sub-carrier, together with its modulation is inserted by the sub-station concerned.

The actual value of the sub-carrier frequencies employed depends mainly on the bandwidth of the frequencies in the messages to be communicated, and also on the final transmission fre-v quency. One probable arrangement for use with speech signals might have fo equal to'15 kc./s., fs

between 100 kc./s. and 500 kc./s. depending mainly on filter designs, and the value of ,fe might be approximately 30 mc./s. if the distance range of communication required is limited.

One possible circuit arrangement of central station is shown schematically in Figure 5, where for simplicity only two channels are shown. The reference 45 represents a generator of pulses of short duration which, as is well known in the art, are electrical waves rich in harmonics of the pulse repetition frequency and these pulses are applied to modulate oscillator 46 of frequency fr so generating the sub-carrier frequencies fi, I: etc. one of which is selected by each amplifier filter 41, 41l etc;

The function of each channel identical sol that one channel only will be described. When it is required to transmit a message by this channel, switches 53, 54, 55 and 58 would be in position T so that filter-ampliiier 41 is connected to unit 46 enabling the required sub-carrierY fixfor example to lbe selected and amplified. The last stage of 41 is arranged so that its output canxbe modulated in a vmodulation amplifier 49 a microphone 5I or any other required` signal generating source.v output from 49 is then caused whose output is coupled radiation;

to aerial arrayTA5U-for In the'casel where it is required vto receiveat vThis modulated sub-carrierv Y to modulate lthe higher principal carrier. frequency oscillator'litiy the central station a signal on this channel switches 53, 54, 55 and 5B are all changed to position R which enables unit vl1 to select its appropriate modulated sub-carrier from a well known demodulator or detector unit 51 which is permanently coupled to oscillator 50 in such a way as to derive therefrom only a very small power, at the same time it must be capable of handling a very large carrier having a very small percentage modulation. The output from 41 is now fed into unit 48 which is another demodulator unit, but in this case adjusted for the sub-carrier frequency so that its output is the required signal, which after amplification by unit 49 (the latter being converted to this use by appropriate switching arrangements, not shown) is fed to its appropriate interpreting apparatus, shown in thiscase as earphones 52.

The arrangement of a sub-station equipment is 'shown schematically in Figure 6 where, when set for receiving a signal, switches 65, B6, 61, 88, 59, 10 and 1| are in position R. The signal transmitted by the central station is received on aerial RT58 and amplified by the high frequency amplifier 58, the output from which is demodulated by 59 in such a way as to give the series of subcarriers from which the appropriate one is selected by filter amplifier 60 which together with units 6| and 62 are similar to their corresponding section 41, 48, 49 at the central station (Fig. 5). The output irom 60 is demodulated by 6I and after amplification by G2 is fed to a translation device, for example, 4to earphones 12.

For transmission of a signal from the substation to the central station, switches 65, 66, B1, 8l,

B9, 10 and 1l are changed to position T and the- 1 sub-carrier corresponding to the channel allotted to the sub-station. This oscillation is then amplied by 69 and modulated, for example, by the output of a microphone 13 or other signal source) after amplification in unit 82 or alternatively, oscillator 63 may be modulated directly. This resultant modulated sub-*carrier from 92 or 63 as the case may be, is then caused to modulate the output of another oscillator 84, either directly, or as shown, in unit 59 which may readily be con.. verted from a demodulator stage to a modulator stage in known manner.- Itis of course essential that the frequency of oscillator 64 be exactly the same as the principal carrier frequency fc. The output from 59 is connected to the aerial RT and so radiated to be received by the central station aerial resulting in a slight modification to the carrier amplitude there but with the addition of the sub-stations side-bands so that the signal contained thereby may be obtained by demodulation, i. e. by unit-51 Figure 5.

principal carrier fe for transmission from the the moduiating input to which may be for example fromv substation is shown in lFigure 'l where oscillator 54 and switch 1l of Figure 6 are replaced by a filter 14 and switch 15. YNo difference is `made to the description for reception, but when transmitting, and all switches. including 15 are in position T, it will be seen that the output of amplifier 58 is` fed to "a filter-14, arranged to be selective enough -t'o pass `only. the 'principal carrier frequency -f (i. e. to pass' no' modulating frequencies) which is thenfejd. to unit 5 9 for modulation and hence` to the aerialfgand input of' amplier 59; Itfwill, therefore, beseenthat the two amplifiers 58 and 59 together with theselective filter 14 arey capable of forming oscillator, generating responding frequency adjustments.

' the distortion caused to the signal being received.

Figure 8 illustrates one possible cause where 18 1 represents the central station and 11 and 18 substations. If the field strengths of the radiations from 18 and 18 are equall at 11 then the carrier component at 11 will be reduced to zero each time the relative transmission paths of the two signals change by multiples of a wavelength, or if the radiations leave 16 and 18 in phase and the distance 18-11 and 18-11 differ by an odd number of half operating wave-lengths. I

An additional circuit arrangement which may be added to the sub-station equipment to overcome this effect is shown in Figure 9 in which the same references as in Figure 6 are used for corresponding umts. When receiving, the output of amplifier 58 is fed to a coupling circuit 18 and thence to unit I8 for the first demodulation.

Unit 18 is such that it enables part of the output of 58 to be fed to a filter 80, capable of passing the carrier frequency fs only. The output from this filter isthen fed to an amplifier 82 and also to a rectifying unit 8|, the output from which is caused toback bias amplifier 8,2. The output from 82 is fed through a phasingcircuit 83 to the coupling unit 18 where its output is eilectively coupled to unit 58 and 80. The adjustment of unit 8| is such that when there is a strong carrier component in the received signal the bias applied to amplier 82 is such as to completely kill it. When however the carrier amplitude falls belowa predetermined yvalue amplifier 82 becomes unbiassed and its output applied to unit 18 malntains the carrier component fed to demodulator 48 at the required level.` This action continues until if the carrier component is reduced to zero .the gain of the amplier 82 together with4 selective circuit 80 phasing circuit 83 and coupling in circuit 18. form an oscillatory circuit, malntaining required input to 58.

Whenl transmitting, all switches are changed to position T, switch 84 reduces the bias applied to amplifier 82 so enabling unit comprising 80, 82, 83 and 18to oscillate so as to generate the required principal carrier amplitude to be fed to unit 59, now modified by means of switches to form a modulator stage. The output from 58 is removed from circuit 18, so that other side-band components are not re-transmitted, causing possiblecancellation with them also.

In an intercommunication system as hereinbefore described, provision may readily be provided for emergency intercommunication between substations by arranging for oscillator 63 and illter 60, Figure 6, to have alternative preset cor- AlsO, if desired arrangement might be made whereby a substationy unit could be converted into a normal type of receiver byarranging for the H. F. amplifier 58 to be converted into a variably tuned unit with oscillator 64 ganged to form a local oscillator for superheterodyne reception using nlter 60 as the intermediate frequency amplifier and in the u skilled in the art and which.

5 case of continuous wave reception oscillator 68 could readily beadjusted to form a. local beat frequency oscillator. v

It will be appreciated that the system according to the invention may be extended to provide lntercommunication other than through the central station but multiplication of the arrangements as at the central station in the foregoing description would be necessary to a degree more or less depending on requirements.

, Since the substations are distributed each is provided with a receiving aerial and also with a local oscillator of frequency fc or alternatively a signal or frequency fc may also be transmitted from the central station, amplified if necessary at the substations and utilised for heterodyning the incoming modulated waves.

Such a provision oi' intercommunication through more than one central station, if intended for normal use may tend to complicate unnecessarily the workingv of the system as a whole, especially if a nels are required.

The adjustment of the individual receivers to any channel is quite a simple operation thus permitting all the receivers to be identical, unless a very large number of channels are employed, in which case it might be advisable to limit the scope of the interchangeability of receivers into groups. l

Whilst several embodiments of the invention have been described, others will occur 'to those fall Within the scope of the appended claims.

What is claimed is:

1. In a radio `communication system for communication between afcentral station and a plurality of sub-stations, transmitting means at the central station'including a plurality of means for producing signal frequency waves. a plurality of means for producing a` sub-carrier frequency Wave, means for producing a principal carrier frequency wave, means for modulating the Wave from each of the sub-carrier producing means with the waves from a signal producing means, means for modulating the principal carrier frequency wave with the modulated sub-carrier frequency Waves and means for Vradiating at least one side-band of the modulated principal carrier frequency wave; at each of said sub-.stations means for producing signal frequency waves, means for producing a sub-carrier frequency Wave, means forproduclng a principal carrier frequency wave, means for modulating the substation sub-carrier frequency wave by the substation signal frequency waves, means for modulating the principal sub-station carrier frequency wave by the modulated sub-station sub-carrier frequency wave, and means for radiating at least one side-band of the modulated principal substation carrier frequency Wave, frequency and side-band of the modulated principal sub-station carrier frequency wave being positioned so that the side-band radiated from the sub-station will cover a frequency band on the opposite side of the principal central station carrier frequency to that occupied by the side-band radiated from said central station.

' 2. In aradio communication system according to claim l, receiving means at the central station including means for heterodyning the received Wave by a sub-carrier-frequency wave, and detecting means, and receiving means at a sub-station including means for heterodyning the revery large number of chanceived wave by the sub-station sub-carrier frequency wave. and detecting means.

8. In a radio communication system according to claim 1, at the central station and at a substation, means for radiating alternately both side-bands of the same principal carrier frequency wave. 1 l l v 4. In a radio comm cation system according to claim 1, at the central station, means for radiating the principal carrier frequency wave, and, at a sub-station, means for producing a principal carrier frequencyA wave including .means for receiving the principal central station carrier frequency wave, and means for deriving therefrom the principal sub-station carrier frequency wave. Y

5. In a radio communication system accordlng to claim 1, at the central station: means for radiating the principal carrier frequency wave, and at a sub-station: means for producing a principal carrier frequency wave including means for receiving the principal central station carrier frequency wave and means for deriving therefrom the principal sub-station carrier frequency wave, means for deriving therefrom the sub-station principal carrier frequency wave, and additional means including a local' oscillator for generating a principal sub-station carrier frequency wave "whenever the received principal central station carrier frequency wave drops below a predetermined value.

6. In a radio communication system according lto claim 1. at the central station: means for radiating the principal carrier frequency wave. and at a sub-station: means for producing a principal carrier frequency wave including meansl for receiving the principal central station car-- rier frequency wave, means for deriving therefrom the principal sub-station carrier frequency wave, and additional means including a local oscillator, and a. feed back path containing thermionic valve means back biased by at least a portion of the received principal central station carrier frequency wave to generate a principal sub-station carrier frequency wave whenever the received principal central station carrier frequency wave drops below a predetermined value.

the latter wave and means for separating the side-bands of the modulated wave to form subcarrier frequency waves.

PRAFULLA CHA'I'IERJEA.

WILFRED HOUGHTON. 

